cotyledon vs frond

cotyledon

noun
  • The leaf of the embryo of a seed-bearing plant; after germination it becomes the first leaves of the seedling. 

  • Each of the patches of villi on the foetal chorion in the placenta of ruminants and some other mammals. 

frond

noun
  • The leaf of a fern, especially a compound leaf. 

  • Any fern-like leaf or other object resembling a fern leaf. 

How often have the words cotyledon and frond occurred in a corpus of books? (source: Google Ngram Viewer )